In a mold opening/closing and mold clamping apparatus for an injection molding machine, mold clamping requires a strong force, while a strong force need not be used continuously for the movement of a movable die plate for opening/closing a mold, but the movement stroke of the movable die plate must be large to take out a product. Previously, the mold clamping and the movement of the movable die plate were performed with the same hydraulic cylinder. In this configuration, however, the hydraulic cylinder was required to have a large diameter and a long stroke, so that the movement of the movable die plate was slow, and the required quantity of oil was large, which caused a problem of poor economy. In recent years, an economical configuration has been used in which a hydraulic cylinder having a small diameter and a long stroke or mechanical movement means in which the rotational force of an electric motor is converted into a force in a straight direction is used for the movement of the die plate, while in the mold clamping process, a hydraulic cylinder having a large diameter and a short stroke is used.
A conventional example described in Japanese Patent Provisional Publication No. 62-87318 (No. 87318/1987) is a mold clamping apparatus including a fixed die plate holding a fixed mold; a movable die plate holding a movable mold; a ball-screw type feed mechanism which advances and retreats the movable die plate toward and from the fixed die plate by means of a driving force of a servomotor; and means for coupling and fixing the movable die plate to a mold clamping piston just before the movable die plate comes close to the fixed die plate and the fixed mold and movable mold are closed.
Also, a mold clamping apparatus described in Utility Model Registration No. 2531522 is provided with a ball-screw type feed mechanism for advancing and retreating the movable die plate toward and from the fixed die plate, which is similar to that mechanism described in the aforementioned Japanese Patent Provisional Publication No. 62-87318, and a tie bar connecting directly to the mold clamping piston for coupling and fixing the movable mold to the fixed mold, and also has means for connecting the movable die plate to the tie bar by engaging half nuts opening and closing in the direction substantially perpendicular to the tie bar provided on the movable die plate with ring grooves formed in the tie bar, so that after the movable die plate is connected to the tie bar, the mold clamping piston is operated hydraulically to pull the movable die plate nearer via the tie bar.
A ball screw of the above-described ball-screw type feed mechanism for advancing and retreating the movable die plate toward and from the fixed die plate has a two-speed mechanism consisting of a mechanism for directly transmitting the rotation of a motor shaft to a ball screw shaft and a mechanism for reducing the rotational speed of the motor shaft and transmitting the rotation thereof to the ball screw shaft, and switching between both mechanisms can be performed by an electromagnetic clutch, so that when a great force is required at the time of mold release, the rotational speed of motor shaft is reduced so that a high torque is applied to the ball screw. In order to balance the load to the movable die plate, two sets of ball-screw type feed mechanisms are provided in symmetrical positions.
Therefore, when the movable die plate is moved, the ball screw shafts must be rotated synchronously.
On the other hand, a toggle type mold clamping apparatus, in which the rotation of an electric motor is transmitted to a screw, and a nut threadedly engaging with the screw is used to convert the rotation into a straight movement, thereby driving a toggle mechanism, so that a fast movement operation of a working end of toggle mechanism is used for mold opening and closing, and a portion generating a great force close to the top dead center of the working end of toggle mechanism is used for mold clamping, has been used for an injection molding machine. This toggle type mold clamping apparatus is simple in construction and can be manufactured at a low cost. However, in order to develop a strong mold clamping force, the whole of the toggle mechanism must inevitably be robust. Therefore, although there is no problem in the case of a small-sized injection molding machine, the structural member is rather excessively large in the case of a large-sized injection molding machine. In this case, component parts such as an electric motor, ball screw, and ball screw nut are expensive as compared with standard parts, which provides an uneconomical equipment.
The conventional ball-screw type feed mechanism, which advances and retreats the movable die plate toward and from the fixed die plate by means of a driving force of a servomotor, is usually provided in a pair at symmetrical positions on both sides of the mold clamping apparatus, and the synchronous speed thereof is controlled so that the rotational speed and rotational position of each ball screw shaft are prevented from shifting. However, when the positions of the paired ball screws shift during operation due to imbalance of load or asynchronism of speed control, or if a ball screw nut mounting portion is tilted by a mounting error occurring when the ball screw nut is assembled to the ball screw shaft, the ball screw shaft comes in unsymmetrical contact with the ball screw nut, and thus a bending force acts on the ball screw shaft from the ball screw nut. Therefore, the ball screw shaft may be damaged, or may be subjected to abnormal wear at an early stage.
Also, two sets of ball screw shafts and ball screw nuts are provided, two sets of servomotors and power transmitting mechanisms, which relate to the ball screw mechanisms, are provided, and the rotation of the two sets of ball screw shafts must be subjected to synchronous operation control, which results in a high cost. Also, when the ball screw shafts and ball screw nuts are installed on both sides of the movable die plate, the mechanism including the motor protrudes from the overall layout of injection molding machine, which increases the occupied floor area. Also, the ball screw nut is fixedly mounted to the movable die plate, the ball screw nut is affected by the deformation of die plate at the time of mold clamping.